Aberrometers measure the refraction of a subject's eye using infrared light.
U.S. Pat. No. 6,550,917, which is incorporated herein by reference, describes an instrument which provides an adjustable telescope in the forward path from the light source to the eye and in the return path from the eye to the wavefront sensor. The adjustable telescope employs a moving stage to move one lens of the telescope with respect to the other, and a feedback arrangement whereby data from the wavefront sensor is employed to control a motor for the moving stage to move the stage to the desired location where the wavefront sensor sees collimated return light from the eye. The moving stage may be a common linear travel stage with stepper (or servo) motor drives and a position encoder. The position of the moving stage may be calibrated so the stage position corresponds to the refractive power of the eye being measured. U.S. Pat. No. 6,550,917 also discloses with respect to FIG. 7 that a variable local length lens may be employed in lieu of a moving stage in order to correct lower order aberrations in the returned light from the eye which is provided to the wavefront sensor
However, U.S. Pat. No. 6,550,917 does not disclose any technique for determining the current focal length of the variable focal length lens when it is employed for measuring a subject's eye. This information is needed to process the wavefront sensor data to determine the aberrations of the subject's eye. Ideally the variable focal lens would operate in a manner such that a control input signal would result in a repeatable setting. However, commercially available devices demonstrate poor repeatability, particularly when the temperature of the operating environment is not stable. Such a situation may occur, for instance, when an instrument is turned on and then heat generated by components in the case causes the internal temperature of the instrument to rise, or when an instrument is built in an open air environment which naturally keeps the instrument cool, but in actual usage a cover is installed that traps heat so the usage conditions are different.